1. Field of the Invention
The present invention is related to a liquid crystal cell structure, and more particularly related to a driving circuit of a liquid crystal on silicon (LCOS) cell structure and a method for controlling the same, which can effectively provide a faster liquid crystal (LC) response time and higher image quality of a display using the liquid crystal on silicon cell structure.
2. Description of the Prior Art
In color displays, there are three major systems for producing different colors and brightness of colors. In the first system, a number of pixels are provided, each pixel transmitting either red, green, or blue light. The pixels are arranged in groups of red, green, and blue. A particular color is achieved in an area by turning on or off the appropriate pixels in that area. For example, if purple is a desired color in an area, the green pixels in that area would remain off and the red and blue pixels would be turned on. The brightness is also controlled by turning on or off the pixels. If bright purple in an area is desired, then all of the red and blue pixels would be turned on in that area. If a darker purple is desired, then some red and blue pixels would turn on partially.
In the second system, similar to the first, a number of pixels are also provided, each pixel transmitting either red, green, or blue light. The pixels are again arranged in groups of red, green, and blue, and again, a particular color is achieved in an area by turning off or on the appropriate pixels in that area. However, brightness is controlled by varying the amount of light being transmitted by a pixel which is on, rather than turning off some of the pixels. As in the first system, if bright purple in an area is desired, then all of the red and blue pixels would be turned on in that area. If a darker purple is desired in an area, then rather than having some of the red and blue pixels remain off in that area, all of the red and blue pixels transmit light, but the amount of light being transmitted from each pixel varies. This second system allows for higher resolution than the first system. Another system similar to this second type is disclosed in which liquid crystals are used as light valves to alter the polarization of incident light on pixels such that more or less of the light striking the pixels will ultimately be transmitted to a display through a beam splitter.
The third system for producing a color display with various colors and brightness of colors is commonly known as field sequential color. In a field sequential color system, each pixel transmits, sequentially in time, red, green, and blue light. When the transmission is fast enough, the human brain fuses all three colors of light into a single color, which is a blend of the colors. Color and brightness of color can be controlled in the time domain. For example, if a bluish, purple color is desired from the pixel during a certain time period, the pixel will transmit blue light longer than red light, and it will transmit no green light. Field sequential color is advantageous in that it allows for very high resolution, since each pixel is independent of its neighbors and can assume any color. However, it has limitations which make it a challenge to commercially exploit, including a requirement for extremely high switching rates. This is in part needed to reduce certain undesirable color effects, including rainbows and color flashes associated with moving objects.
The liquid crystal display (LCD) has been considered a very important screen display device for holding display data. One conventional display device for holding display data in units of pixels is sometimes referred to as a TFT (Thin-Film-Transistor) active matrix type liquid crystal display device or TFT LCD device which has many pixels arranged in a matrix. A color pixel is formed usually by combining three pixels. However, the size of a conventional TFT LCD device is relatively large. In view of its large size, the conventional TFT LCD device can only be fabricated on a piece of large glass substrate with poly-silicon thin-film transistors.
As a consequence, in recent years, there has been developed a new class of mini-display based on a single crystal silicon substrate. The newer mini-displays can be manufactured using current CMOS technological processes, which can provide better yield and a higher level circuit integration than the existing TFT LCD devices. These mini-displays are referred to as liquid crystal on silicon (LCOS) devices, which are in essence miniature version of the TFT LCD device. In fact, the LCD portion of the mini-display is quite similar to that of the TFT LCD device, but is made on a much smaller scale (e.g. on top of a silicon chip). The image on top of the mini-display is typically magnified for viewing by an optical system. Dependent upon the particular application, the optical system may be quite complex.
The specific applications for the LCOS device can be generally classified into three major categories. Firstly, the current important application is for use in the area of very large screen projectors in which the size of the display device is about thirty inches or more. Secondly, the application is for utilization as a desktop computer monitor in which the size of the display device is in the range between seventeen and thirty inches. Lastly, the third application for the LCOS device is for use as a portable personal display unit.
FIG. 1 is an exploded, perspective view of a conventional LCOS display device 10, which includes a silicon chip substrate 12, a plurality of bonding pads 14 disposed on its peripheral edges, and a display cell array 16 located in the central part of the substrate 12. A circuit area 18 is positioned around the cell array 16. A top glass cover 20 having a seal-ring 22 is securely mounted over the display cell and the circuit area, with a liquid crystal (LC) layer sandwiched therebetween. Because of the non-transparency of the silicon substrate, the structure is suitable for the reflective mode. FIG. 2a is a schematic circuit diagram of a conventional LCOS cell structure having one transistor and one capacitor. The LCOS cell structure 204 comprises a transistor T1 having a gate, a drain, and a source, a storage capacitor Cs, a mirror electrode 206, a top electrode 208, and a liquid crystal 209 sandwiched between the mirror electrode and the top electrode. The transistor T1 has its gate connected to the corresponding wordline WL. The transistor T1 has its source connected to a bitline BL. The transistor T1 has its drain connected to the mirror electrode 206 and one end of the storage capacitors Cs together, and the other end of the storage capacitor Cs is connected to a ground potential. The cell structure is also called 1C1M (one capacitor one mirror) cell. There are two reason illustrated by the figure that the LCOS cell structure is smaller than the conventional TFT-LCD device. First, a gate insulating film used for forming the storage capacitor is extremely thin and thus the surface area thereof can be made smaller than the conventional TFT device. Secondly, the access transistor can be fabricated at any location under the mirror plate without causing a blockage of light.
As a consequence, the proprietor develops a technique to generate a color image from the LCOS display device according to the three color display described above. For example, in a large screen projection type color display device, there are always three LCOS devices used together with precision optics so as to process the three colors, corresponding to red (R), green (G), and blue (B). On the other hand, in a portable type display device where there is a concern for size, weight, and/or cost, only one LCOS device is used on which a color image must be generated. In order to achieve the color image for the portable display, there are three-pixel cells used within the LCOS device which is then covered with color filters for the corresponding RGB colors. However, the pixel array area becomes then approximately three times larger which is unsuitable for a high yield on the CMOS silicon technology. In addition, the use of color filters makes the standard CMOS silicon process more complex and thus increases cost.
In order to overcome these disadvantages, there have been developed a prior art technique of creating a color image in a one-pixel cell which is referred to as a “Field Sequential” (FS) method. This FS method writes RGB data to each of the one-pixel cells in the pixel array in three sequential operations at three times the clock rate. During each of the three sequential operations, a corresponding RGB light source is driving synchronously.
FIG. 2b is a timing diagram showing the sequences of operation of the LCOS cell structure of FIG. 2a. A frame time is divided into three sub-frame times: a red-field, green-field and blue-field time; and each of the red-field, green-field and blue-field time divided into only three pipe times consisting of an LC response time, a light-strobing time, and a reset/preset time (field switching time too short to be neglected). The video image quality depends on the light-strobing time so that a higher quality video image is allowed by increasing the light-strobing time.
This FS method will function acceptably as long as the response time of the liquid crystal LC is sufficient enough. If this is the case, then the three sequential colors will be effectively combined into a single color image. Therefore, the effective LC response time for each of colors is ⅓ of the frame time reduced by the amount of time it takes to write a color field. However, a serious problem arises when this FS method is utilized in a relatively large display device. This is because there may not be enough time for the LC to respond. For instance, a standard 60 frames/sec video signal has a total time of 16.67 ms for displaying a frame. In the case of the three-pixel cells where the RGB colors can be processed in parallel, each color has a full frame time so as to process and then display the video data. On the other hand, in the FS operation each of the colors has only ⅓ of the frame time or 5.56 ms in order to write data to the storage capacitors, to then wait for the LC to respond, and to then finally strobe the pixel array with the corresponding RGB light source. The length of time for light strobing (LS) the pixel array will determine the brightness of the color image. Consequently, in order to achieve a high image quality in the LCOS device operated with the FS method, it has become necessary to effectively increase the LS time.
FIG. 3a is a schematic circuit diagram of a conventional LCOS cell structure having six transistors and three capacitors. FIG. 3b is a timing diagram showing the sequences of operation of the LCOS cell structure of FIG. 3a. The detail technology can be seen in U.S. Pat. No. 6,421,037 issued to C. L. Chen on Jul. 16, 2002 entitled “Silicon-Chip-Display cell structure”, which is incorporated herein by reference.
Additionally, the requirement of liquid crystal response rate in the display mechanism of LCD using FS model is faster than that of liquid penal of iconmeter LCD or LCD using three-piece projection, since only one black and white panel need to display in color. If the LCD is driven at 180 Hz (60*3), a frame time will be divided into three sub-frame times for red, green and blue color. Therefore, the liquid crystal response rate will be preferably as fast as possible. In general, the LCD panel using FS model is often selected from the LCD panel of the ferro-electric liquid crystal model, which is using ferro-electric liquid crystal material, or TN model (pitch <1.5 Uum), which is used for manufacturing thin LCD panel, so as to achieve the colored requirement. However, both of the LCD panel techniques have problems of lower reliability for the product, especially the manufactures continuously devote to overcome the problems of lower reliability in the application of ferro-electric liquid crystal model on LCOS. There are only samples of the ferro-electric liquid crystal model for demonstration. Therefore, the LCD product of ferro-electric liquid crystal model having fast response will be not present in the near future, and the main stream product is still TN model LCD. For the TN model LCD, in addition to selecting the liquid crystal material with low viscosity coefficient, the method for increasing the response rate is reducing the cell gap of the liquid crystal. However, reducing the cell gap of the liquid crystal results in problem of reducing the manufacturing reliability since the class of the clean room must increase after reducing the cell gap, and the problem of particle seriously affect the reliability. Moreover, the cell gap needs more precision, and the tolerance of the cell gap reduces. Furthermore, if the liquid crystal material with high Dn needs to be selected, the selectivity of the liquid crystal material is small. Therefore, reducing of the cell gap not only reduces the manufacturing reliability but also increases the manufacturing cost.
Therefore, based on above reason, there is needed to provide a driving circuit of liquid crystal on silicon (LCOS) liquid crystal cell in a reasonable and high reliability range of current process providing faster LC response time and higher image quality of a display using the LCOS cell structure.